The present invention is directed to a system and method for controlling a motor torque output. More particularly, the invention relates to a system and method for controlling a motor torque output by monitoring an operating condition of a hydrostatic motor.
A pump having a variable displacement capability is well known in the industry to drive a hydrostatic motor. In a machine having a variable displacement pump and a hydrostatic motor, such as an excavator or a loader, the pump provides fluid to the hydrostatic motor and drives the motor in the forward or reverse direction. The speed of the hydrostatic motor is controlled by varying the displacement of the pump.
A variable displacement pump generally includes a drive shaft, a rotatable cylinder barrel having multiple piston bores, pistons held against a tiltable swashplate, and a valve plate. When the swashplate is tilted relative to the longitudinal axis of the drive shaft, the pistons reciprocate within the piston bores to produce a pumping action. Each piston bore is subject to intake and discharge pressures during each revolution of the cylinder barrel.
Similarly, a hydrostatic motor includes an output drive shaft, a rotatable cylinder barrel having multiple piston bores, pistons disposed in the bores, a swashplate, and a valve plate. The pistons reciprocate within the piston bores to produce a pumping action. Each piston bore is subject to intake and discharge pressures during each revolution of the cylinder barrel. The pumping action by the pistons rotates the cylinder barrel and the output drive shaft, thereby providing a motor torque output.
In a hydrostatic motor, the total fluid flow through the motor to produce the torque output is geometrically proportional to the sum of the velocities of the individual pistons between the bottom dead center (BDC) and the top dead center (TDC) positions. A hydrostatic motor often has an odd number of pistons and piston bores in the cylinder barrel. When the motor has, for example, nine pistons and corresponding pistons bores, there may be five pistons pressurized at a certain rotational position of the cylinder barrel and four pistons pressurized at another rotational position. This difference in the number of the pressurized pistons in a revolution of the piston barrel results in variation in the motor torque output, also known as a motor torque ripple.
The torque ripple is not limited to motors having an odd number of pistons. In a motor having an even number of pistons, the numbers of the pressurized pistons may change as the motor rotates, and this also can result in a motor torque ripple. In addition to the above described causes for the torque ripple, minor geometrical changes and port timing can contribute to the motor torque ripple.
The motor torque ripple from the hydrostatic motor may be transmitted to other components in the machine and produce undesirable noise and vibrations. Moreover, the motor torque ripple can exert a stress on other components in the machine, thereby decreasing machine life. Motor torque ripple is more troublesome at low motor speeds or low motor piston pass frequencies, which occur at the start of the motor, and can result in a jerk motion at the start of the machine.
Various attempts have been made to reduce noise in hydraulic systems. For example, U.S. Pat. No. 5,492,451 discloses a system and method for attenuating fluid-borne noise from a pump. The system includes a mechanism for sensing a flow ripple produced by the pump, and a mechanism for transmitting a signal to a negative ripple generator to cancel the flow ripple.
However, the disclosed system does not control the displacement of the pump or motor to reduce a motor torque ripple at a motor torque output.
Thus, it is desirable to provide a torque control system that provides flexible control of a pump or motor to reduce a motor torque ripple in real time. The present invention is directed to solving one or more of the shortcomings associated with prior art designs.
In one aspect, a method is provided for controlling a torque output of an hydrostatic motor in fluid communication with a pump. At least one of the motor and pump has variable displacement capabilities. The method includes monitoring at least one motor condition of the hydrostatic motor and determining a displacement signal corresponding to the monitored motor condition. The variable displacement capabilities are controlled based on the displacement signal to reduce a torque ripple in the torque output of the hydrostatic motor.
In another aspect, a system is provided for controlling a torque output of a hydrostatic motor in fluid communication with a pump. At least one of the motor and pump has variable displacement capabilities. The system includes a sensor assembly for monitoring at least one motor condition of the hydrostatic motor and an actuator configured to control the variable displacement capabilities of the motor or pump. A controller is electrically coupled to the sensor assembly and the actuator, and the controller is configured to determine a displacement signal corresponding to the motor condition and control the variable displacement capabilities based on the displacement signal to reduce a torque ripple of the torque output of the hydrostatic motor.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.